## Multi-tone parallel coherent matched detection for demultiplexing of superchannels |

Optics Express, Vol. 21, Issue 16, pp. 18602-18610 (2013)

http://dx.doi.org/10.1364/OE.21.018602

Acrobat PDF (1804 KB)

### Abstract

This paper presents multi-tone parallel coherent matched detection that orthogonally detects superchannels without crosstalk between neighboring channels. The receiver consists of multiple sets of multi-tone coherent matched detector employed in parallel. In each detector, the received superchannels signal is homodyne mixed in multi frequency with locally generated multi-tone optical frequency comb; detected is a signal set that has the amplitude and phase exactly matched with the local comb. By launching orthogonal sets of local comb to the multiple parallel coherent matched detectors, the received superchannels are orthogonally downconverted to the baseband frequencies keeping the amplitude and phase information of all channels included. With an aid of *n* × *n* transform matrix, all channels are separately recovered from the downconverted signal sets. The system does not rely on any optical filters for channel demultiplexing and separation, with increased flexibility in wavelength arrangement. In addition, the parallel configuration equivalently enhance the bandwidth of the coherent matched detector keeping the speed in each tributary channel as high as possible. In this paper, it is experimentally demonstrated that even-odd interleaved 23 × 20-Gb/s QPSK superchannels are orthogonally demultiplexed and detected by two-tone coherent matched detection.

© 2013 OSA

## 1. Introduction

*i.e.*1 [Baud/Hz] per single polarization [1

1. W. Shieh and C. Athaudage, “Coherent optical orthogonal frequency division multiplexing,” Electron. Lett. **42**(10), 587–589 (2006) [CrossRef] .

1. W. Shieh and C. Athaudage, “Coherent optical orthogonal frequency division multiplexing,” Electron. Lett. **42**(10), 587–589 (2006) [CrossRef] .

3. Q. Yang, Y. M. Y. Tang, and W. Shieh, “Experimental demonstration and numerical simulation of 107-Gb/s high spectral efficiency coherent optical OFDM,” J. Lightwave Technol. **27**(3), 168–176 (2009) [CrossRef] .

## 2. Principles

*n*-parallel coherent matched detector and (b)

*n*×

*n*transform matrix section, where

*n*is the number of the channels received with this setup.

*n*sets of coherent matched detector, where the received superchannels signal is split in

*n*and individually led into the coherent matched detector. In each coherent matched detector, the signal is homodyne mixed with a locally generated optical multi-frequency carriers (hereafter, we call it local comb) in a similar way with optical sampling systems sampled with local pulse trains [7

7. K. Kikuchi, K. Igarashi, Y. Mori, and C. Zhang, “Demodulation of 320-Gbit/s Optical Quadrature Phase-Shift Keying Signal with Digital Coherent Receiver Having Time-Division Demultiplexing Function,” in the 2008 Optical Fiber Communication Conference (OFC 2008), OtuO4 (2008) [CrossRef] .

9. N. Fontaine, G. Raybon, B. Guan, A. Adamiecki, P. Winzer, R. Ryf, A. Konczykowska, F. Jorge, J. Dupuy, L. L. Buhl, S. Chandrashekhar, R. Delbue, P. Pupalaikis, and A. Sureka, “228-GHz Coherent Receiver using Digital Optical Bandwidth Interleaving and Reception of 214-Gbd (856-GB/s) PDM-QPSK,” in 38th European Conference on Optical Communication (ECOC 2012), Th.3.A.1 (2012).

*n*frequency components with the equal amplitude and with the constant frequency spacing of

*B*[Hz], which is same as the symbol rate of each tributary channel,

*B*[Baud]. Through the multi-frequency homodyne mixing in each detector, the

*i*-th channel,

*d*at the wavelength of

_{i}*λ*(for

_{i}*i*= 1, ⋯ ,

*n*), is mixed with the corresponding frequency component of the local comb and all channels [

*d*

_{1}, ⋯ ,

*d*] are simultaneously downconverted to the baseband frequencies, while other beating components between the received channels,

_{n}*d*, and the frequency components of local comb at

_{i}*λ*(

_{j}*i*≠

*j*) are filtered out with a low-pass filter (LPF) followed by the homodyne mixer. By this downconversion process, all channels are constructively added and matched detected if the detected waveform matches the condition of [Δ

*θ*

_{1}, ⋯ ,Δ

*θ*] = [

_{n}*θ*

_{s,1}−

*θ*

_{l,1}, ⋯ ,

*θ*

_{s,n}−

*θ*

_{l,n}] = [0, ⋯ ,0], where

*θ*

_{s,i}and

*θ*

_{l,i}are the optical phase offset of the received signal and local comb at the wavelength of

*λ*; Δ

_{i}*θ*is relative phase difference between them.

_{i}*r*

_{1}, ⋯ ,

*r*], are mixtures of data channels, [

_{n}*d*

_{1}, ⋯ ,

*d*]. To recover the original data sets from [

_{n}*r*

_{1}, ⋯ ,

*r*], we need to apply

_{n}*n*×

*n*transform matrix,

*M*, as shown in section (b) in Fig. 1, which should be the inverse of the transfer matrix of the coherent matched detectors,

*A*.

*M*, just focusing on two-tone (

*n*= 2) and three-tone (

*n*= 3) coherent matched detection cases, for simplified explanation. Figure 2 shows orthogonal sets of local comb for

*n*= 2 and

*n*= 3. Two coherent matched detectors give

*π*phase difference to the phase offset between the two frequency components, in the two-tone detected case. If the local comb in the first coherent matched detector has phase offsets of [

*θ*

_{l,1}+

*π*/4,

*θ*

_{l,2}−

*π*/4], those of local comb in the second detector should be [

*θ*

_{l,1}−

*π*/4,

*θ*

_{l,2}+

*π*/4]. In the three-tone detected case, 2

*π*/3 phase difference are given to the phase offset of each frequency component between the three sets of coherent matched detectors. The phase offsets of local comb in the first and third detectors should be [

*θ*

_{l,1}+ 2

*π*/3,

*θ*

_{l,2},

*θ*

_{l,3}− 2

*π*/3] and [

*θ*

_{l,1}− 2

*π*/3,

*θ*

_{l,2},

*θ*

_{l,3}+ 2

*π*/3] if those in the second detector are [

*θ*

_{l,1},

*θ*

_{l,2},

*θ*

_{l,3}]. These phase offsets can be translated to the temporal delays of (2

*B*)

^{−1}for

*n*= 2 and (3

*B*)

^{−1}for

*n*= 3, respectively. With these orthogonal local comb sets, the transfer matrices of the coherent matched detectors,

*A*

_{2}for

*n*= 2 and

*A*

_{3}for

*n*= 3, are described as

*θ*

_{1}, Δ

*θ*

_{2}and Δ

*θ*

_{3}stand for phase offsets at tributary channels relative to corresponding local comb line (

*i.e.*Δ

*θ*≡

_{i}*θ*

_{s,i}−

*θ*

_{l,i}); Δ

*ϕ*

_{1}, Δ

*ϕ*

_{2}, Δ

*ϕ*

_{3}are phase differences between the received signal and local combs. To derive these transfer matrices, we assumed that influence from the second neighboring channels is negligible, which is satisfied if the LPFs applied in the coherent matched detectors have good suppression around the frequency region. Practically, we can choose root-raised-cosine (RRC) or any other filters with roll-off factors steep enough. (If filters with non-steep slope are chosen, dimension of the matrices need to be enhanced to deal with the influence from the second neighboring channels, increasing the complexity of the system.) The 2 × 2 and 3 × 3 transform matrices

*M*

_{2}and

*M*

_{3}, corresponding to

*A*

_{2}and

*A*

_{3}respectively, yield the inverse of the transfer matrices:

*θ*and Δ

_{i}*ϕ*are always drifting due to carrier phase noise of lasers; thus, the elements of matrices requires real-time update. Multi-input-multi-output (MIMO) updating algorithm [11

_{i}11. Y. Han and G. Li, “Coherent optical communication using polarization
multiple-input-multiple-output,” Opt. Express **13**(19), 7527–7534
(2005) [CrossRef] .

12. S. Randel, R. Ryf, A. Sierra, P. Winzer, A. Gnauck, C. Bolle, R. Essiambre, D. Peckham, A. McCurdy, and R. Lingle, “6×56-Gb/s mode-division multiplexed transmission over 33-km few-mode fiber enabled by 6×6 MIMO equalization,” Optics Express **19**, 16,697–16,707 (2011) [CrossRef] .

*θ*. On the other hand, Δ

_{i}*ϕ*is recovered and canceled out by using the algorithm for carrier phase estimation used in single-carrier homodyne systems. Through the digital signal processing, slowly varying drift of Δ

_{i}*θ*and Δ

_{i}*ϕ*are always tracked and updated. This practically makes it easier to implement the coherent matched detectors. First, optical carrier phase drift of the received signal against LO combs can be canceled without using optical phase locking technique, similarly with single-carrier digital homodyne detection. The signals and local combs input into coherent matched detectors are also not required to be phase locked each other, which means that typically available optical hybrid couplers can be used for the parallel coherent matched detection without integrating them nor stabilizing optical phase between the local combs. The MIMO equalization implemented in the transform matrix section also helps to cancel the impairments of the system, misalignment of the delay between the local comb, residual chromatic dispersion, and so on. For example, it is numerically confirmed that almost no penalty is observed even if there is 5 % misalignment in the optical delay between the local combs, in the 2 × 2 case.

_{i}## 3. Experiments

*n*= 2, where we assume that superchannels interleaved from even and odd channels are received with a two-parallel two-tone coherent matched detector.

*M*

_{2}, after the resampling at 20 GSa/s with a locally recovered clock signal. Each element of the transform matrix consists of a fractional FIR filter with a tap length of 15; co-efficients of the center (7-th) tap are initiated in accordance with Eq. 2; the tap coefficients are always updated using MIMO equalizing technique, watching them not to be diverged. This section has a role to cancel the crosstalk between the channel, compensating for signal distortion due to impairments like residual chromatic dispersion and clock misalignment, and so on. After the processing in the transform matrix section, the carrier phase is recovered with 4-th power algorithm and the QPSK signal in each channel is decoded for bit-error-rate evaluation. All these functions are implemented on offline DSP.

*M*

_{2}. The constellations are monitored at the Ch. 0 as pointed in Fig. 4(a). It is clearly seen that two channels are simultaneously downconverted to the baseband frequencies causing large crosstalk, in this situation. If we turn on the

*M*

_{2}and keep updating the matrix elements, on the other hand, the two signal sources are clearly separated, as shown in Figs. 4(c) and 4(d), where (c) and (d) correspond to the constellations of Ch. 0 and Ch. 1, respectively.

## 4. Conclusion

## Acknowledgment

## References and links

1. | W. Shieh and C. Athaudage, “Coherent optical orthogonal frequency division multiplexing,” Electron. Lett. |

2. | S. Jansen, I. Morita, T. Schenk, N. Takeda, and H. Tanaka, “Coherent optical 25.8-Gb/s OFDM transmission over 4160-km SSMF,” J. Lightwave Technol. |

3. | Q. Yang, Y. M. Y. Tang, and W. Shieh, “Experimental demonstration and numerical simulation of 107-Gb/s high spectral efficiency coherent optical OFDM,” J. Lightwave Technol. |

4. | A. Sano, H. Masuda, E. Yoshida, T. Kobayashi, E. Yamada, Y. Miyamoto, F. Inuzuka, Y. Hibino, Y. Takatori, K. Hagimoto, T. Yamada, and Y. Sakamaki, “30×100 Gb/s all-optical OFDM transmission over 1300 km SMF with 10 ROADM nodes,” in 33th European Conference on Optical Communication (ECOC 2007), PDP 1.7 (2007). |

5. | S. Chandrasekhar, X. Liu, B. Zhu, and D. Peckham, “Transmission of a 1.2-Tb/s 24 carrier No-Guard-Interval Coherent OFDM Supperchannel over 7200-km of Ultra-Large-Area Fiber,” in 35th European Conference on Optical Communication (ECOC 2009), PD2.6 (2009). |

6. | T. Xia, G. Wellbrock, K. Huang, M. Huang, E. Ip, N. Ji, D. Qian, A. Tanaka, Y. Shao, T. Wang, Y. Aono, and T. Tajima, “21.7 Tb/s Field Trial with 22 DP-8QAM/QPSK Optical Superchannels Over 1,503-km Installed SSMF,” in the 2008 Optical Fiber Communication Conference (OFC 2012), PDP5D.6 (2012). |

7. | K. Kikuchi, K. Igarashi, Y. Mori, and C. Zhang, “Demodulation of 320-Gbit/s Optical Quadrature Phase-Shift Keying Signal with Digital Coherent Receiver Having Time-Division Demultiplexing Function,” in the 2008 Optical Fiber Communication Conference (OFC 2008), OtuO4 (2008) [CrossRef] . |

8. | K. Fischer, R. Ludwig, L. Molle, C. S. Langhorst, C. Leonhardt, A. Matiss, and C. Schubert, “Digital Coherent Receiver Based on Parallel Optical Sampling,” in 36th European Conference on Optical Communication (ECOC 2010), Th10.A.4 (2010) [CrossRef] . |

9. | N. Fontaine, G. Raybon, B. Guan, A. Adamiecki, P. Winzer, R. Ryf, A. Konczykowska, F. Jorge, J. Dupuy, L. L. Buhl, S. Chandrashekhar, R. Delbue, P. Pupalaikis, and A. Sureka, “228-GHz Coherent Receiver using Digital Optical Bandwidth Interleaving and Reception of 214-Gbd (856-GB/s) PDM-QPSK,” in 38th European Conference on Optical Communication (ECOC 2012), Th.3.A.1 (2012). |

10. | T. Sakamoto, T. Kawanishi, and M. Izutsu, “Asymptotic formalism for ultraflat optical frequency comb generation using a Mach-Zehnder modulator,” Opt. Lett. |

11. | Y. Han and G. Li, “Coherent optical communication using polarization
multiple-input-multiple-output,” Opt. Express |

12. | S. Randel, R. Ryf, A. Sierra, P. Winzer, A. Gnauck, C. Bolle, R. Essiambre, D. Peckham, A. McCurdy, and R. Lingle, “6×56-Gb/s mode-division multiplexed transmission over 33-km few-mode fiber enabled by 6×6 MIMO equalization,” Optics Express |

**OCIS Codes**

(060.1660) Fiber optics and optical communications : Coherent communications

(060.4510) Fiber optics and optical communications : Optical communications

**ToC Category:**

Fiber Optics and Optical Communications

**History**

Original Manuscript: March 19, 2013

Revised Manuscript: June 10, 2013

Manuscript Accepted: June 15, 2013

Published: July 29, 2013

**Citation**

Takahide Sakamoto, Guo-Wei Lu, and Tetsuya Kawanishi, "Multi-tone parallel coherent matched detection for demultiplexing of superchannels," Opt. Express **21**, 18602-18610 (2013)

http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-21-16-18602

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### References

- W. Shieh and C. Athaudage, “Coherent optical orthogonal frequency division multiplexing,” Electron. Lett.42(10), 587–589 (2006). [CrossRef]
- S. Jansen, I. Morita, T. Schenk, N. Takeda, and H. Tanaka, “Coherent optical 25.8-Gb/s OFDM transmission over 4160-km SSMF,” J. Lightwave Technol.26(1), 6–15 (2008). [CrossRef]
- Q. Yang, Y. M. Y. Tang, and W. Shieh, “Experimental demonstration and numerical simulation of 107-Gb/s high spectral efficiency coherent optical OFDM,” J. Lightwave Technol.27(3), 168–176 (2009). [CrossRef]
- A. Sano, H. Masuda, E. Yoshida, T. Kobayashi, E. Yamada, Y. Miyamoto, F. Inuzuka, Y. Hibino, Y. Takatori, K. Hagimoto, T. Yamada, and Y. Sakamaki, “30×100 Gb/s all-optical OFDM transmission over 1300 km SMF with 10 ROADM nodes,” in 33th European Conference on Optical Communication (ECOC 2007), PDP 1.7 (2007).
- S. Chandrasekhar, X. Liu, B. Zhu, and D. Peckham, “Transmission of a 1.2-Tb/s 24 carrier No-Guard-Interval Coherent OFDM Supperchannel over 7200-km of Ultra-Large-Area Fiber,” in 35th European Conference on Optical Communication (ECOC 2009), PD2.6 (2009).
- T. Xia, G. Wellbrock, K. Huang, M. Huang, E. Ip, N. Ji, D. Qian, A. Tanaka, Y. Shao, T. Wang, Y. Aono, and T. Tajima, “21.7 Tb/s Field Trial with 22 DP-8QAM/QPSK Optical Superchannels Over 1,503-km Installed SSMF,” in the 2008 Optical Fiber Communication Conference (OFC 2012), PDP5D.6 (2012).
- K. Kikuchi, K. Igarashi, Y. Mori, and C. Zhang, “Demodulation of 320-Gbit/s Optical Quadrature Phase-Shift Keying Signal with Digital Coherent Receiver Having Time-Division Demultiplexing Function,” in the 2008 Optical Fiber Communication Conference (OFC 2008), OtuO4 (2008). [CrossRef]
- K. Fischer, R. Ludwig, L. Molle, C. S. Langhorst, C. Leonhardt, A. Matiss, and C. Schubert, “Digital Coherent Receiver Based on Parallel Optical Sampling,” in 36th European Conference on Optical Communication (ECOC 2010), Th10.A.4 (2010). [CrossRef]
- N. Fontaine, G. Raybon, B. Guan, A. Adamiecki, P. Winzer, R. Ryf, A. Konczykowska, F. Jorge, J. Dupuy, L. L. Buhl, S. Chandrashekhar, R. Delbue, P. Pupalaikis, and A. Sureka, “228-GHz Coherent Receiver using Digital Optical Bandwidth Interleaving and Reception of 214-Gbd (856-GB/s) PDM-QPSK,” in 38th European Conference on Optical Communication (ECOC 2012), Th.3.A.1 (2012).
- T. Sakamoto, T. Kawanishi, and M. Izutsu, “Asymptotic formalism for ultraflat optical frequency comb generation using a Mach-Zehnder modulator,” Opt. Lett.32(11), 1515–1517 (2007). [CrossRef] [PubMed]
- Y. Han and G. Li, “Coherent optical communication using polarization multiple-input-multiple-output,” Opt. Express13(19), 7527–7534 (2005). [CrossRef]
- S. Randel, R. Ryf, A. Sierra, P. Winzer, A. Gnauck, C. Bolle, R. Essiambre, D. Peckham, A. McCurdy, and R. Lingle, “6×56-Gb/s mode-division multiplexed transmission over 33-km few-mode fiber enabled by 6×6 MIMO equalization,” Optics Express19, 16,697–16,707 (2011). [CrossRef]

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